1. Field of the Invention
The present invention relates to a process for the preparation of spontaneously crosslinked alkali metal acrylate polymers having a high water-absorbing property and a high sanitary safety.
2. Description of the Prior Art
Paper products for absorbing excretions from human bodies, such as sanitary napkins and paper diapers, have been used for many years, but their properties are not completely satisfactory. Most of the defects of these paper products are due to the insufficient water-absorbing property of the paper. In paper, water is absorbed in clearance spaces between fibers by capillary action. Accordingly, as the amount of water absorbed increases, the bulk of the paper becomes extremely large. Further, the majority of the absorbed water is squeezed out by application of external pressure.
As products intended for overcoming this defect, there have heretofore been proposed various composite products formed by combining a water-absorbing polymer with paper, pulverized pulp, absorbent cotton or the like. For example, U.S. Pat. No. 3,670,731 discloses that a low bulk product having a high water-absorbing property even under the application of external pressure, is prepared by distributing a partially crosslinked acrylamide-sodium acrylate copolymer in an absorbent material.
The water-absorbing polymer that can be used in this field is obtained by partially cross-linking a water-soluble polymer so that the resulting polymer can be swollen by water, but it does not dissolve in water. In view of the intended use, the water-absorbing polymer must be sanitary and safe for contact with the human body, and it is required that the water-absorbing polymer can be manufactured at a low cost.
In general, as disclosed in U.S. Pat. No. 3,669,103, water-swellable but water-insoluble polymers, such as partially crosslinked polyacrylamide and poly(alkali metal acrylate), that can be used as water-absorbing polymers, are prepared by radical copolymerization of an aqueous solution of a corresponding monomer and a crosslinking monomer such as a divinyl or diallyl compound. The resulting massive hydrous gel is dried and then pulverized. Further, a water-soluble polymer prepared in advance may be crosslinked by radiation.
As an example of a water-soluble polymer having a high sanitary safety that can be used in the above-mentioned field, namely a basic skeleton of a water-absorbing polymer, there can be mentioned poly(sodium acrylate), which is permitted for use as a food additive. In order to obtain a water-absorbing polymer, this polymer must be crosslinked. It is not preferred to copolymerize sodium acrylate using a divinyl or diallyl compound as the crosslinking monomer because the sanitary safety of a polymer prepared in that way has not been proved. Post-crosslinking by radiation is excellent from the viewpoint that an extraneous chemical compound is not introduced into the polymer. However, in the latter case, complicated preparation equipment is required, and the preparation process involves a large investment and the manufacturing cost is extremely high.
It is known that in the polymerization of acrylic acid or sodium acrylate, a partially or highly crosslinked polymer can be prepared under some preparation conditions. For example, it is reported that in the aqueous solution polymerization of sodium acrylate, because the monomer concentration is high, at a pH of 4 to 6 the induction period is very long and after this long induction period, rapid polymerization accompanied with bumping due to accumulation of the heat of polymerization takes place and the entire mixture is converted to a granular gel which is poorly soluble in water (see "Acrylic Acid and Its Polymer," volume I, page 51, written by Eizo Ohmori and published by Shokodo in 1973). It is also reported that a polymer obtained by water-in-oil emulsion polymerization of acrylic acid is partially insoluble in water (see Japanese Patent Publication No. 10644/59). However, the primary object of these known processes is to obtain a soluble poly(sodium acrylate). In these processes, the crosslinking reaction is an abnormal and unwanted reaction, and it is one of the technical problems in the conventional polymerization processes to devise procedures to eliminate the occurrence of such abnormal crosslinking reaction.
We have noted this abnormal reaction and searched for polymerization conditions effective to cause such abnormal reaction to occur on a reliable, reproducible basis so that poly(sodium acrylate) can be uniformly and spontaneously crosslinked so as to produce a polymer product having a high swelling property but containing only a very small water-soluble portion.
More specifically, we prepared polymers according to the teachings of the above textbook reference and Japanese patent publication. As described in the Comparative Examples given hereinafter, the degree of swelling of a polymer obtained by aqueous solution polymerization at a high concentration of sodium acrylate is not very high and the amount of the water-soluble portion of the polymer is large. In other words, the polymer is highly sticky when it is dispersed in water. Moreover, since this reaction progresses very vigorously and rapidly, it is very difficult to conduct this polymerization process on an industrial scale while controlling the reaction appropriately. In the case of the water-in-oil emulsion polymerization of acrylic acid, the resulting polymer is obtained in the state of a dispersion in water, and when it is neutralized with sodium hydroxide, a large quantity of a water-soluble polymer portion is formed and the resulting product is not satisfactory as a water-absorbing polymer. At any rate, in view of the intended use of the polymer, a water-absorbing polymer must be substantially neutral, and the resulting polyacrylic acid must be neutralized with an alkaline substance. Therefore, in the process in which a crosslinked polyacrylic acid is prepared and it is then neutralized, a gel having a high viscosity must be treated and it must be pulverized. However, the steps of removing the solvent and water from a highly viscous gel and pulverizing and powdering the resulting solid are quite difficult and disadvantageous from the industrial viewpoint.